It is a common practice for chemicals such as those used for cleaning and sanitizing to be purchased as concentrated liquids. The chemicals are mixed with water to achieve the desired usage concentration. A variety of proportioning dispensers have been developed to achieve this. These dispense mixtures at use concentration. The dispensers often employ venturi-type devices sometimes called eductors to proportion the chemical and deliver this for use. Water traveling through the central portion of the venturi creates suction which draws the chemical into the water stream. The amount of chemical educted is controlled by a metering orifice in the chemical feed line.
The concentrations desired in this type of chemical dispensing varies greatly ranging from 1:1 to over 1:1000. The devices also must function with a wide range of water pressures, temperatures and dissolved minerals and gases. In some of these conditions, the eductor functions much like a classical flow venturi, while in other they are more like a jet pump. The devices are mechanically simple, generally without moving parts, but small details of the construction have important influence on their performance.
It is usually desirable to operate these dispensers with water provided directly from the public water supply. In this situation, the dispensers are subject to the regulations of the public water departments who are concerned about preventing any possibility of the chemical concentrates syphoning or flowing back into the water system. Venturi-type chemical eductors with an air gap for back siphoning protection for dispensing applications are disclosed in the Sand U.S. Pat. Nos. 5,253,677 and 5,159,958, both of which are assigned to the Assignee of the present invention. The essential geometry of a venturi is that of a constriction and then a downstream enlargement of a contained stream of fluid. According to Bernoulli's theory, suction is created at the point where the flow channel widens. The operation of the venturi requires that the entering fluid stream have a certain amount of flow energy. For an air gap eductor, this means that the stream must cross the air gap and enter the venturi while developing an appreciable pressure within the entrance of the venturi.
The geometry which creates this function includes an inlet orifice for directing a first fluid, for example, water, that has a diameter larger than the smallest orifice within the eductor venturi. The eductor venturi includes a larger diameter mixing chamber downstream of the smallest venturi orifice. A second fluid, for example, a liquid chemical, is pulled by suction through a second inlet into the mixing chamber and mixed with the first fluid. A venturi diffuser extends from the mixing chamber and flares outwardly to conduct the mixture of the first and second fluids, that is, the water and the chemical to an eductor outlet. A spray shield is located between the eductor air gap and the eductor venturi and blocks spray from reentering the air gap.
While the above chemical eductors work satisfactorily, there are several disadvantages to their designs. First, under some circumstances, current spray shield designs, may not optimally direct spray or collected water. Further, current spray shield designs require that the periphery of the spray shield have a water tight connection with the internal walls of the eductor body. That construction requirement adds complexity and cost to the process of manufacturing the eductor. In addition, the designs of current eductors are complex. The dimensional tolerances are relatively small, and the components of the chemical eductor require machining. The machined components are then assembled by welding, adhesives or other techniques to form the eductor. Therefore, the manufacture of the chemical eductor requires expensive capital equipment and highly skilled labor, and further, is complex and time consuming, all of which adds substantial cost to the eductor unit.